The present invention relates to expandable particles of a vinyl chloride resin composition, and more particularly to expandable particles of a vinyl chloride resin composition for beads-foaming which can be easily molded to foamed articles of the vinyl chloride resin composition which have complicated shapes with smooth surfaces by pre-expanding the expandable particles, filling a mold with the pre-expanded particles, and carring out molding in the mold.
As processes for preparing foamed articles of a vinyl chloride resin composition, there have hitherto been known (a) a process in which a chemical blowing agent which gasifies by heat decomposition is admixed with a vinyl chloride resin and the mixture is heat-molded to foamed articles by an extruder or an injection molding machine; (b) a process in which a vinyl chloride resin and a plasticizer are mixed to prepare a plastisol, and the plastisol is mechanically foamed, or a chemical blowing agent is admixed with the plastisol and the mixture is melted and foamed by heating; (c) a process in which a mixture of a vinyl chloride resin and a chemical blowing agent is previously roll-molded at a temperature of not higher than the decomposition temperature of the chemical blowing agent and then foamed by heating; (d) a process in which a mold is filled with a vinyl chloride resin and a chemical blowing agent (if necessary, an easily volatile organic blowing agent and an organic solvent which swells or softens the resin are additionally used), heated under pressure to melt, cooled down, and heated again to foam; (e) a process in which an extruder is charged with a vinyl chloride resin into which a volatile blowing agent is previously impregnated or an extruder is charged with a vinyl chloride resin and then a blowing agent is introduced into the extruder to carry out extrusion foaming; and the like.
However, the above-mentioned processes (a) to (e) have the following various defects.
By the processes (a) to (c), rigid or semi-rigid foamed articles having high expansion ratios cannot be prepared; as to the process (d), the costs of end products are great since the process is a batchwise process of which producing steps are complicated and a lot of time is required for preparing the foamed articles; and as to the process (e), though rigid or semi-rigid foamed articles having high expansion ratios can be relatively easily prepared, foamed articles having complicated shapes cannot be prepared due to extrusion foaming.
Vinyl chloride resin foamed articles have various advantages such as good noninflammability, good toughness and low cost over polystyrene foamed articles. However, there have not been provided expandable particles of vinyl chloride resin which can be molded in high expansion ratios by beads-foaming technique.
An object of the present invention is to provide expandable particles of a vinyl chloride resin composition for beads-foaming which can be easily molded to foamed articles having desired complicated shapes with smooth surfaces.
The above and other objects of the present invention will become apparent from the description hereinafter.